At Home Depot you can buy 3 piece ply known as “sanderlay”(sp). It is approximately 1/8" thick, but only comes in 8’ sheets. It is pretty inexpensive considering the number of stringers that can be gotten out of a sheet. For anything over eight foot, I scarf two pieces together by using a V joint. One piece is cut at a point and fitted into a V on the other piece. This can be easily done by gluing the stringer to only one half of the Blank. Then after it dries, glue the second half onto the stringered half. If done properly most of the time the joint isn’t noticeable. If it is; barely so. It’s essentially a glue lam beam. A sheet of Sandlery Ply is usually about $25 at HD.
McDing, did you notice tearing when you plane the ply. I did a few with 1/8" ply and stringer would tear when I planed the board. I still use it for perimeter rails. Very strong for the weight, yet pliable. It is a mahogany wood here, and often labeled 1/8" door skin. Very inexpensive.
We are lucky to have a lot of Redwood fencing here. It comes in 5/8" thickness and up to 8" wide up to 8’ long. Only problem is the quality isn’t great and you have to sort through a whole bin just to get one or two good pieces.
The stuff I buy is not Mahogany Door skin. Last time I was in a Depot I noticed they had it in two thicknesses. It planed well, but I was careful to pick a couple of sheets that had no voids at the center layer. I don’t know what the type of wood is, but it looks very much like like Basswood or Alder. Definitely not Mahogany. And I am familiar with door skins. Not the same. Door Skins are usually Mahogany, Birch, Oak etc.
